Seasonal changes in feeding habits and diet of the cockle Cerastoderma glaucum (Mollusca, Bivalvia) were analysed using carbon (δ 13C) and nitrogen (δ 15N) stable isotopes. I aimed to investigate the role of benthic and pelagic sources in the diet of this dominant infaunal bivalve on a western Mediterranean sandy bottomed pond. Adult C. glaucum and all potential organic sources (particulate and sedimentary organic matter, seagrass, macroalgae, heterotrophic detritus) were collected and analysed for δ 13C and δ 15N. In total 5 dominant organic sources were found, ranging between –21.0 and –8.0 ‰ for δ 13C and from 3.0 to about 7.0% for δ 15N. C. glaucum assimilated fraction ranged between –14.0/–15.0% and 6.0/9.0% respectively, for carbon and nitrogen, while its estimated fractionation seasonally varied between 1 and 1.5 for carbon and was about 3 for nitrogen. C. glaucum reflected the isotopic composition of its food while maintaining, throughout the year, a fairly constant isotopic composition in its assimilated fraction, despite the high variability in isotopic composition of available sources.
